infuse_iot: database: handle parallel access

Handle parallel access by multiple threads when the offline cache is
used. The `shelve` module does not appear to handle this by default.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Author: JordanYates

src/infuse_iot/database.py

@@ -4,6 +4,8 @@
 import binascii
 import pathlib
 import shelve
+import threading
+from contextlib import contextmanager
 
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import x25519
@@ -77,6 +79,7 @@ def __init__(
         self._local_root: x25519.X25519PrivateKey | None = None
         self._local_root_public: bytes | None = None
         self._cache_path = cache_path
+        self._cache_lock = threading.Lock()
         if local_root:
             with local_root.open() as f:
                 private_key = serialization.load_pem_private_key(f.read().encode("utf-8"), password=None)
@@ -130,18 +133,23 @@ def observe_secondary_remote_public_key(self, infuse_id: int, secondary_pub_key:
         dev.secondary_device_key_id = binascii.crc32(self._local_root_public + dev.device_public_key) & 0xFFFFFF
         dev.local_shared_key = self._local_root.exchange(device_public_key)
 
+    @contextmanager
+    def _with_cache(self):
+        with self._cache_lock as _lock, shelve.open(str(self._cache_path)) as cache:
+            yield cache
+
     def _update_cache(self, infuse_id: int, device_pub_key: bytes, shared_key: bytes):
         if self._cache_path is None:
             return
         infuse_id_str = f"{infuse_id:016x}"
-        with shelve.open(str(self._cache_path)) as cache:
+        with self._with_cache() as cache:
             cache[infuse_id_str] = {"public_key": device_pub_key, "shared_key": shared_key}
 
     def _from_cache(self, infuse_id: int, device_pub_key: bytes) -> bytes | None:
         if self._cache_path is None:
             return None
         infuse_id_str = f"{infuse_id:016x}"
-        with shelve.open(str(self._cache_path)) as cache:
+        with self._with_cache() as cache:
             state = cache.get(infuse_id_str, None)
             if state is None or state["public_key"] != device_pub_key:
                 return None